Load lifting devices such as cranes, and especially mobile cranes, often use telescoping booms to achieve the necessary lift height. A telescoping boom is made up of multiple sections which telescope with respect to one another to change the overall length of the boom. The telescoping boom of a portable crane is often extended by one or more hydraulic devices, typically cylinders, acting on the sections of the boom. Fluid is supplied to, or removed from, a hydraulic cylinder to cause a piston to move within the hydraulic cylinder. Movement of the piston enables the boom of the load lifting device to extend or contract.
A natural phenomenon is known to occur in telescoping booms, caused by thermal expansion and the subsequent contraction of fluid in the hydraulic cylinder supporting the boom. This natural phenomenon may be observed when a load lifting device is operated for extended periods of time causing the fluid in the hydraulic cylinder to heat up and subsequently cool down. Hydraulic fluid expands when it is heated and contracts when it is cooled. A load lifting device may be left idle for a period of time during which the fluid cools down. During such time the elevation angle of the boom above horizontal may be relatively low. In this instance, when the fluid cools it contracts but the boom may sometimes not retract because of frictional forces acting between individual boom sections. This effect varies depending on the particular boom configuration, the level of friction between individual sections of the boom, lubrication of the boom sections, and other possible environmental factors. Thus, the telescoping boom could remain extended even though the boom sections are not fully supported by fluid in hydraulic cylinders.
In the described situation the relative positions of the boom sections might be supported by friction between individual sections of the boom. If the lifting machine operator elevates the boom from the low elevation angle position the boom will remain at the same boom length for a certain range of elevation angles. However, if the operator continues to elevate the boom, the boom will eventually reach an elevation angle where the weight of the boom sections or a combination of the weight of the sections and any other load overcomes the friction between boom sections. At this point, the boom may retract until the column of fluid in the cylinders again fully supports the boom sections. As can be appreciated, this uncommanded boom retraction is undesirable. The present invention provides a system and a method for avoiding this undesirable situation.